can a single wire support full duplex transmission?

Question

(1) can a single wire, e.g. copper wire, support full duplex transmission?

(2) still a single single wire, can it support transmission of multiple bits at a time in one direction?

(3) USB supports serial transmission, theoretically a single wire is enough. But why it has so many wires? ---I am not an electrical engineer, this question might sound too simple.

Answer 1

A 'single wire' as such isn't possible. At the very least, you require a signal wire plus a ground connection for single-ended signaling, or a twisted pair for differential signaling. Fiber can use a single strand, of course. And there's wireless.

1. yes, using either echo cancellation (e.g. 1000BASE-T), frequency-division duplex (e.g. xDSL) or (for fiber) wavelength-division duplex (e.g. 1000BASE-BX10)
2. yes, using MLT, PAM or a more complex multi-bit modulation (esp. QPSK or QAM)
3. copper very often uses differential signaling over twisted pairs for cost reasons, USB is off topic here though